aio: sanitize ki_list handling

Instead of handcoded non-null checks always initialize ki_list to an
empty list and use list_empty / list_empty_careful on it.  While we're
at it also error out on a double call to kiocb_set_cancel_fn instead
of ignoring it.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Jeff Moyer <jmoyer@redhat.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>

fs/aio.c

@@ -553,13 +553,12 @@ void kiocb_set_cancel_fn(struct kiocb *iocb, kiocb_cancel_fn *cancel)
 	struct kioctx *ctx = req->ki_ctx;
 	unsigned long flags;
 
-	spin_lock_irqsave(&ctx->ctx_lock, flags);
-
-	if (!req->ki_list.next)
-		list_add(&req->ki_list, &ctx->active_reqs);
+	if (WARN_ON_ONCE(!list_empty(&req->ki_list)))
+		return;
 
+	spin_lock_irqsave(&ctx->ctx_lock, flags);
+	list_add_tail(&req->ki_list, &ctx->active_reqs);
 	req->ki_cancel = cancel;
-
 	spin_unlock_irqrestore(&ctx->ctx_lock, flags);
 }
 EXPORT_SYMBOL(kiocb_set_cancel_fn);
@@ -1039,7 +1038,7 @@ static inline struct aio_kiocb *aio_get_req(struct kioctx *ctx)
 		goto out_put;
 
 	percpu_ref_get(&ctx->reqs);
-
+	INIT_LIST_HEAD(&req->ki_list);
 	req->ki_ctx = ctx;
 	return req;
 out_put:
@@ -1107,7 +1106,7 @@ static void aio_complete(struct kiocb *kiocb, long res, long res2)
 		file_end_write(file);
 	}
 
-	if (iocb->ki_list.next) {
+	if (!list_empty_careful(&iocb->ki_list)) {
 		unsigned long flags;
 
 		spin_lock_irqsave(&ctx->ctx_lock, flags);